1. Field of the Invention
The present invention relates to a current limiting circuit, and more particularly, to protection against overcurrent in an output transistor.
2. Description of Related Art
A power supply semiconductor is known as a circuit that controls supply of power to a load from a power supply. In the power supply semiconductor, an N-channel MOS transistor is used as an output transistor for a high-side switch, for example.
The power supply semiconductor for use in an LSI (Large Scale Integration) for automotive electrical equipment needs to be protected so as not to be damaged, even if an abnormality such as short-circuiting of a harness to ground occurs.
For example, if the covering of a harness for connecting a load is peeled off and the harness comes into contact with the body (ground) of a vehicle, the load is short-circuited. As a result, excessive power may be applied to the power supply semiconductor. In order to prevent the power supply semiconductor from being damaged in such a case, the power supply semiconductor is typically mounted with an overcurrent protection circuit. As an example of the overcurrent protection circuit, a current limiting circuit is known.
The current limiting circuit performs a feedback control to prevent an overcurrent from flowing through the power supply semiconductor. In other words, the current limiting circuit controls the gate voltage of the output transistor so as to prevent an overcurrent from flowing between the source and drain of the output transistor.
A vehicle-mounted semiconductor device is used over a fairly wide temperature range in view of the environment in which the device finds use. Accordingly, it is necessary for the current limiting circuit to accurately limit the overcurrent flowing through the output transistor without being affected by a temperature change.
FIG. 10 shows a current limiting circuit disclosed in Japanese Unexamined Patent Application Publication No. 2008-236528. The circuit shown in FIG. 10 includes an output transistor 201, a wire 217, a booster circuit 215, diode groups 221 and 222, an amplifier 203, a transistor 202, and current sources 204 and 205. The diode groups 221 and 222, the amplifier 203, the transistor 202, and the current sources 204 and 205 constitute a control circuit 220. The output transistor 201 has a switch function for controlling a current flowing from a power supply 211 to a load 212. The control circuit 220 detects a potential difference generated between both ends (nodes A and B) of the wire 217, and limits a current flowing through the output transistor 201 when the potential difference is greater than a predetermined value.
The technique disclosed in Japanese Unexamined Patent Application Publication No. 2008-236528 has a feature that an overcurrent flowing through the output transistor 201 is limited with high accuracy even when the resistance component of the wire 217 varies due to a temperature change, by utilizing the fact that a temperature coefficient at a potential difference between the two diode groups 221 and 222 and a temperature coefficient of an electrical resistivity of the wire 217 through which a load current flows are equal to each other.
Further, Japanese Unexamined Patent Application Publication No. 2005-39573 and U.S. Pat. No. 7,626,792 B2 disclose an overcurrent detecting circuit as a related art (see FIG. 4 of Japanese Unexamined Patent Application Publication No. 2005-39573 and FIG. 6 of U.S. Pat. No. 7,626,792 B2). The overcurrent detecting circuit disclosed in Japanese Unexamined Patent Application Publication No. 2005-39573 includes an output transistor Q1, a sense transistor Q2 provided in parallel with the output transistor Q1, a detection resistor RS connected in series with the sense transistor Q2, and comparators (transistors 9, 10, Q3, and Q4) that compare a potential generated by the detection resistor RS with a threshold voltage and output an overcurrent detection signal.
Furthermore, U.S. Pat. No. 5,579,193 discloses an overcurrent protection circuit as a related art (see FIG. 2 of U.S. Pat. No. 5,579,193). The overcurrent protection circuit disclosed in U.S. Pat. No. 5,579,193 includes an output transistor 110, a sense transistor 112 provided in parallel with the output transistor, a sense resistor R1 connected in series with the sense transistor 112, a comparator 120 that compares a potential generated by the sense resistor R1 and a reference value, and a limiting circuit 20 that controls a conduction state of the output transistor based on a comparison result of the comparator 120. It is also disclosed that the sense resistor R1 may be a metal resistor.